If you would like to help with FlightGear's development, it's a good idea to subscribe yourself to the FlightGear development mailing list. Use the list or the FlightGear issue tracker to get in touch with fellow FlightGear core developers.
Visit Category:Core development projects to see a list of currently ongoing projects!
See Category:Developer Plans to see a list of developer plans for the upcoming release.
See
request for comments for a list of long-standing issues and challenges affecting FlightGear's evolution and overall development progress (admittedly a little outdated meanwhile).